1. Field of the Invention
The present invention relates generally to air-to-fuel ratio control systems for internal combustion engines, and more particularly, to a system for controlling an air-to-fuel ratio of a fuel mixture provided for combustion in an internal combustion engine employed in a vehicle to be of a value different from the value of the theoretical air-to-fuel ratio when the operation of the engine meets predetermined specific conditions.
2. Description of the Prior Art
In an internal combustion engine employed in a vehicle and accompanied with a ternary catalyst converter for purifying the exhaust gas therefrom, it is generally preferable to keep the air-to-fuel ratio of a fuel mixture supplied to a combustion chamber at the value of the theoretical air-to-fuel ratio or a value approximating thereto, because the purification effect for the exhaust gas is advanced theoretically under the condition wherein the fuel mixture has the theoretical air-to-fuel ratio. Because of this, an air-to-fuel ratio control system was proposed in which the air-to-fuel ratio of the fuel mixture is subjected to a feedback-control performed in response to the output of an air-to-fuel ratio sensor, such as an oxygen sensor located in an exhaust passage from the engine for detecting oxygen gas contained in the exhaust gas, so as to be of a value in a relatively narrow range including the value of the theoretical air-to-fuel ratio.
On the other hand, in view of improving fuel consumption in the engine, it is sometimes desired to cause the fuel mixture to have an air-to-fuel ratio larger than the theoretical air-to-fuel ratio so as to provide a lean mixture or, an air-to-fuel ratio smaller than the theoretical air-to-fuel ratio so as to provide a rich mixture which may be appropriate for the situation wherein the engine is required to produce relatively large power for accelerating the vehicle. But, because the air-to-fuel ratio sensor previously proposed and generally used in the feedback-control for the air-to-fuel ratio of the fuel mixture is so constituted as to produce an output which varies in level to high from low or vice versa in the condition wherein the fuel mixture has the theoretical air-to-fuel ratio, it is quite difficult to keep the air-to-fuel ratio of the fuel mixture at a value larger or smaller than the value of the theoretical air-to-fuel ratio through such a feedback control performed in response to the output of the air-to-fuel ratio sensor as discussed above.
Accordingly, in the air-to-fuel ratio control system proposed previously, when the fuel mixture is maintained with an air-to-fuel ratio larger or smaller than the theoretical air-to-fuel ratio, the air-to-fuel ratio of the fuel mixture is subjected to a open-loop control in which the fundamental quantity of fuel is calculated on the strength of engine speed and engine load which varies in proportion to air flow in an intake passage to the engine or air pressure arising in the intake passage, and revised to produce a final quantity of fuel, and then the fuel mixture supplied to the combustion chamber of the engine is controlled so as to contain the fuel of the final quantity. However, in such a system wherein the open-loop control for the air-to-fuel ratio of the fuel mixture is performed in the manner mentioned above, the control is easily influenced by the secular change in the characteristic of the engine or variations in the driving conditions of the vehicle so as to be unstable or deteriorate in accuracy, so that it is difficult to properly keep the desired air-to-fuel ratio of the fuel mixture.
Then, as described in, for example, the Japanese patent application published before examination with the publication No. 57/105530 on July 1, 1982, another air-to-fuel ratio control system, in which a control for keeping the fuel mixture having a desired air-to-fuel ratio different from the theoretical air-to-fuel ratio is performed almost stably without being easily influenced by the secular change in the characteristic of the engine, has been proposed. In such a system, the feedback control for keeping the air-to-fuel ratio of the fuel mixture being of a value in a relatively narrow range including the value of the theoretical air-to-fuel ratio is carried out and the learning function for obtaining a supplemental feedback quantity of fuel used for revising therewith the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is performed under that feedback control. Then, when the open-loop control for keeping the air-to-fuel ratio of the fuel mixture being of the value different from the value of the theoretical air-to-fuel ratio is carried out, the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is revised based on the supplemental feedback quantity of fuel obtained through the learning function, so that the fuel mixture is maintained so as to have a desired air-to-fuel ratio different from the theoretical air-to-fuel ratio and approximations thereof.
In such a previously proposed air-to-fuel ratio control system wherein the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is revised with a result of the learning function performed under the feedback control when the open-loop control for the air-to-fuel ratio of the fuel mixture is carried out, a single supplemental feedback quantity of fuel obtained through the learning function performed in a certain operating condition of the engine is used for revising the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined in the whole operating condition of the engine. However, because the quantity of the fuel actually desired to be supplied to the combustion chamber is varied in accordance with the operating condition of the engine, although the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is revised based on the supplemental feedback quantity of fuel obtained through the learning function so that the desired air-to-fuel ratio of the fuel mixture is obtained in the certain operating condition of the engine, for example, in the condition wherein the engine works with a relatively light load under the open-loop control, errors in the open-loop control may arise so that the desired air-to-fuel ratio of the fuel mixture can not be obtained properly in other operating conditions of the engine, for example, in the condition wherein the engine works with a relatively heavy load.
Accordingly, there is proposed such a modified control system as mentioned below. In the modified control system, the feedback control for keeping the air-to-fuel ratio of the fuel mixture having a value in a relatively narrow range including the value of the theoretical air-to-fuel ratio is carried out in each of a plurality of partitions of the operating condition of the engine (hereinafter, each of such partitions will be referred to as an operating partition) and the learning function for obtaining a supplemental feedback quantity of fuel used for revising therewith the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is performed under that feedback control in each operating partition. The supplemental feedback quantity of fuel obtained through the learning function in each operating partition is memorized as a resultant of the learning function in a corresponding memorizing area partitioned in a memory device in accordance with, or corresponding to the operating partitions. Then, whenever the engine is started working, in each operating partition, the resultant of the learning function in the corresponding memorizing area is renewed. Subsequently, the open-loop control for the air-to-fuel ratio of the fuel mixture is carried-out, so that the quantity of fuel by which the fuel actually supplied to the combustion chamber is determined is revised with the resultant of the learning function obtained in response to the operating condition of the engine.
With this proposed air-to-fuel ratio control system, in each operating partition, the fuel mixture is caused to have the theoretical air-to-fuel ratio or approximations thereof under the feedback control performed for obtaining the supplemental feedback quantity of fuel through the learning function, and also caused to have a desired air-to-fuel ratio different from the theoretical air-to-fuel ratio or approximations thereof under the open-loop control. For example, in the case where the open-loop control for keeping the fuel mixture having an air-to-fuel ratio larger than the theoretical air-to-fuel ratio or approximations thereof is performed, as shown with a dot-dash line in FIG. 1 with the axis of abscissa representing the time (t) and the axis of ordinate representing the air-to-fuel ratio (A/F), the air-to-fuel ratio of the fuel mixture is maintained at the value of the theoretical air-to-fuel ratio (14.7) and approximations thereof during each of periods H.sub.1, H.sub.2, H.sub.3, H.sub.4, . . . in respective operating partitions Za, Zb, Zc, Zd, . . . , in which the feedback control is performed, and then kept to be of the desired value (18) larger than the value of the theoretical air-to-fuel ratio and approximations thereof during each of periods G.sub.1, G.sub.2, G.sub.3, G.sub.4, . . . successive respectively to the periods H.sub.1, H.sub.2, H.sub.3, H.sub.4, . . . in respective operating partitions Za, Zb, Zc, Zd, . . . , in which the open-loop control is performed, that is, the change in the fuel mixture to lean from theoretical occurs in each of the operating partitions Za, Zb, Zc, Zd, . . . . Accordingly, the air-to-fuel ratio of the fuel mixture is suddenly varied at each transition between the period during which the feedback control is performed and the period during which the open-loop control is performed, so that undesirable variations in torque produced by the engine may arise frequently.
Further, in the case mentioned above, it is required to have the period, such as each of H.sub.1, H.sub.2, H.sub.3, H.sub.4, . . . , in which the feedback control for obtaining the supplemental feedback quantity of fuel through the learning function is performed before the corresponding period, such as each of G.sub.1, G.sub.2, G.sub.3, G.sub.4, . . . , in which the open-loop control for keeping the fuel mixture having the desired air-to-fuel ratio larger than the theoretical air-to-fuel ratio or approximations thereof is performed in each operating partition, such as each of Za, Zb, Zc, Zd, . . . , and therefore the start of the open-loop control may be undesirably delayed in each operating partition.